Remove "ssh:" prefix from functions names.

* src/auth.c, src/auth.scm, src/channel-func.c,
  src/channel-type.c, src/channel.scm, src/key-func.c,
  src/key-type.c, src/key.scm, src/session-func.c,
  src/session-type.c, src/session.scm, src/version.c,
  src/version.scm: Remove "ssh:" prefix from functions names.
  Update comments.
src/session-func.c: Don't use the TYPE macro.

* src/session-func.c (session_options): Don't use TYPE macro to
  get names of SSH constants.
  (set_option): Likewise.
  (TYPE): Remove the macro.
NEWS: Update.